1. Field of the Invention
The present invention relates to FM modulation devices, and more particularly to a modulation device that produces wider-band frequency-modulated signal (hereinafter referred to as FM modulated signal) by using a semiconductor laser element.
2. Description of the Background Art
FIG. 3 is a block diagram showing a structure of a conventional FM modulation device. In FIG. 3, the conventional FM modulation device includes a signal source 301, a laser element for FM modulation (hereinafter referred to as an FM laser element) 302, a first optical waveguide portion 303, a local light source 304, a second optical waveguide portion 305, and a photodetection portion 306. The structure and operation of this conventional FM modulation device will now be described in detail.
The signal source 301 outputs an electrical signal as the original signal for FM modulation. The FM laser element 302 is formed of a semiconductor laser element, for example. With constant injection current, it oscillates light at a wavelength .lambda.1. When the injection current is amplitude-modulated, its outputted light is modulated in intensity and also in oscillation wavelength (optical frequency), and it outputs optical-frequency-modulated signal around the wavelength .lambda.1. The first optical waveguide portion 303 guides the optical signal outputted from the FM laser element 302. The local light source 304 outputs light at a wavelength .lambda.0, which is different from the oscillation wavelength .lambda.1 of the FM laser element 302 by a given quantity .DELTA..lambda.. The second optical waveguide portion 305 guides the unmodulated light from the local light source 304. The two lights guided by the first and second optical waveguide portions 303 and 305 are combined and inputted to the photodetection portion 306. The photodetection portion 306 is formed of a photodiode having a square-law detection characteristic, for example. It has the property of outputting variations in optical intensity of the inputted light as variations in current amplitude (hereinafter referred to as IM-DD components), and it also has the property of, when two lights of different wavelengths are inputted thereto, producing a beat component of the two lights at a frequency corresponding to the wavelength difference (this operation is called a heterodyne detection). Accordingly, the photodetection portion 306 outputs the beat signal of the outputted optical signal from the FM laser element 302 and the outputted light from the local light source 304 at a frequency corresponding to the wavelength difference .DELTA..lambda. between the two lights. This beat signal is an FM modulated signal resulting from the original electrical signal from the signal source 301.
As described above, the FM modulation device that converts an electrical signal into an optical-frequency-modulated signal by utilizing the characteristic that a semiconductor laser element, for example, varies its oscillation wavelength in accordance with the injection current (called wavelength chirp), and further converting it into an electrical signal by an optical heterodyne detection can easily realize wide-band FM modulation performance with very large frequency deviation, such as cannot be realized with popular electric modulation devices, through the use of appropriate FM laser element 302 and local light source 304. This enables FM modulation to wide-band signals, like multi-channel frequency-division-multiplexed signals used in CATV etc.
Currently, the CATV most generally uses the AM (Amplitude Modulation) transmission system. This method requires the transmission method to present a very good noise characteristic (e.g., SNR: 51 dB or higher). On the other hand, the FM (Frequency Modulation) transmission method has the advantage that it does not require a very good noise characteristic, unlike the AM transmission method, although it requires a wider transmission band. Accordingly, when AM-FDM (Frequency Division Multiplex) signal currently used in CATV is simultaneously converted into an FM signal by using an FM modulation device having the above structure and then transmitted, the AM signal requiring high SNR can be transmitted through a transmission line with inferior noise characteristic. (For example, refer to K. Kikushima et al., "150-km Non-Repeated 60-Channel AM-Video Transmission Employing Optical Heterodyne AM/FM Converter," ECOC'95, Th.L.3.1, Brussels, 1995.)
However, since the outputted optical signal from the FM laser element 302 contains an optical intensity modulation component, the FM modulated signal produced by the FM modulation device shown in FIG. 3 contains not only a frequency variation but also an average-value variation component, which considerably deteriorates signal quality in FM demodulation. This problem will now be described in detail.
FIG. 4 is a schematic diagram showing relations between spectral shapes and waveforms of the wide-band FM modulated signal produced in a conventional FM modulation device. It is assumed here that the signal source 301 outputs a frequency-division-multiplexed multi-channel signal as the original signal for FM modulation. When the optical signal outputted from the FM laser element 302 has only the optical frequency modulation component, the produced wide-band FM modulated signal is an ideal FM modulated signal as shown in FIG. 4(a), whose spectrum is in a symmetrical shape about the carrier at the frequency fc corresponding to the difference in oscillation wavelength between the FM laser element 302 and the local light source 304, (.lambda.1-.lambda.0). On the other hand, as has been explained referring to FIG. 3, when the optical signal outputted from the FM laser element 302 is modulated not only in optical frequency but also in optical intensity (IM modulation), the waveform of the modulated signal contains the amplitude variation (IM modulation component) as shown in FIG. 4(b) (a signal like this is called an FM+IM signal, hereinafter), whose spectral shape is not symmetrical. Further, in the conventional FM modulation device shown in FIG. 3, the optical intensity modulation component is square-law-detected in the photodetection portion 306, and the component (IM-DD component) is superimposed upon the FM modulated signal. Then, as shown in FIG. 4(c), the average-value level of the generated FM+IM signal will vary with the IM-DD component.
Described next is the waveform deterioration caused in FM demodulation when the FM modulated signal contains the average-value variation. FIG. 5 shows a typical structure of a demodulation circuit for wide-band FM modulated signal. This diagram also shows waveforms which will be presented in individual portions when an ideal FM modulated signal is inputted to this FM demodulation circuit.
In FIG. 5, the FM demodulation circuit includes a discrimination portion 501, a delay portion 502, an AND gate 503, and a low-pass filter (LPF) 504. The discrimination portion 501 is formed of logic element, such as AND gate, for example. It discriminates the inputted FM modulated signal with a given threshold Vref to convert (to pulse) the FM modulated signal into a pulse signal (logic signal). The discrimination portion 501 has two output ports. One of its outputted signals is directly inputted to the AND gate 503, and the other one is delayed by a given quantity in the delay portion 502 and then inputted to the AND gate 503. The AND gate 503 produces AND signal of the two inputted pulse signals. The LPF 504 passes only the low-frequency components in the outputted signal from the AND gate 503. The amplitude variation component obtained in this way uniquely corresponds to the frequency variation component in the inputted FM modulated signal. The FM modulated signal can thus be demodulated.
However, as stated above, the conventional FM modulation device cannot produce an ideal FM modulated signal. It outputs an FM modulated signal that contains average-value variation (IM-DD component). Accordingly, when the inputted FM modulated signal is discriminated with a given fixed threshold Vref as in the FM demodulation circuit shown in FIG. 5, it is difficult to correctly discriminate the signal, as can be seen from FIG. 6. Then the demodulated signal will provide deteriorated waveform.
As described above, the FM modulation device with the structure shown in FIG. 3 has its unique problem that the IM-DD component generated by the photodetection portion 306 and superimposed upon the wide-band FM modulated signal decreases the discriminating accuracy in FM demodulation and causes waveform deterioration. In other words, when a wide-band FM modulated signal is produced by utilizing the optical frequency modulating operation by a semiconductor laser element and the heterodyne detection, optical intensity modulation is produced as well as optical frequency modulation, which causes average-value variation in the wide-band FM modulated signal. Then FM demodulation cannot be correctly performed.